U.S. patent number 7,645,092 [Application Number 11/756,387] was granted by the patent office on 2010-01-12 for treatment of in ground chromium ore processing residue.
This patent grant is currently assigned to CH2M Hill, Inc.. Invention is credited to Thomas E. Higgins.
United States Patent |
7,645,092 |
Higgins |
January 12, 2010 |
Treatment of in ground chromium ore processing residue
Abstract
In methods for treatment of in ground chromium ore processing
residue contamination, ferrous sulfide is provided as a
substantially insoluble material in the residue deposit. The
ferrous sulfide accordingly may remain substantially in place, in
the pores of the soil or residue, even over long periods of time,
regardless of underground water movement or diffusion. As a result,
the ferrous sulfide may act continuously to chemically reduce and
remove contamination. As hexavalent chromium diffuses from the soil
or residue, it is reduced by the ferrous sulfide. The ferrous
sulfide may be placed into the underground deposit by injecting a
ferrous salt solution into the deposit, waiting for a precipitation
to elapse, and then injecting a sulfide solution into the
deposit.
Inventors: |
Higgins; Thomas E. (Reston,
VA) |
Assignee: |
CH2M Hill, Inc. (Englewood,
CO)
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Family
ID: |
46327976 |
Appl.
No.: |
11/756,387 |
Filed: |
May 31, 2007 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20070225542 A1 |
Sep 27, 2007 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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11278009 |
Mar 30, 2006 |
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60732511 |
Nov 2, 2005 |
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Current U.S.
Class: |
405/128.75;
588/412; 588/319 |
Current CPC
Class: |
B09C
1/08 (20130101); C02F 1/5236 (20130101); B09C
1/002 (20130101); B09C 2101/00 (20130101); C02F
2101/22 (20130101); C02F 2103/06 (20130101); C02F
1/70 (20130101) |
Current International
Class: |
B09C
1/00 (20060101) |
Field of
Search: |
;405/128.1,128.5,128.75
;588/319,412 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
US.P.T.O. Final Office Action mailed Sep. 10, 2007 in U.S. Appl.
No. 11/361,325. cited by other .
Earth Science Services, "Biogeochemical Reductive Dechlorination,"
Dec. 2003. cited by other .
Higgins, Thomas E., et al., "Treatment of Plating Wastewaters by
Ferrous Reduction, Sulfide Precipitation, Coagulation and Upflow
Filtration," Proceedings of the 36th Industrial Waste Conference,
Ann Arbor Science--The Butterworth Group, May 12-14, 1981, pp.
462-471. cited by other .
USPTO Final Office Action mailed Sep. 10, 2007 in U.S. Appl. No.
11/361,325. cited by other .
The International Search Report and the Written Opinion of the
International Searching Authority, mailed Nov. 16, 2007 in
International Application No. PCT/US06/60425. cited by other .
U.S. Appl. No. 11/361,325, filed Feb. 24, 2006--189442/US/2. cited
by other .
Notice of Abandonment dated Mar. 28, 2008, U.S. Appl. No.
11/361,325, 2 pages. cited by other .
Final Office Action dated Sep. 10, 2007, U.S. Appl. No. 11/361,325,
9 pages. cited by other .
Amendment dated Jun. 12, 2007, U.S. Appl. No. 11/361,325, 14 pages.
cited by other .
Office Action dated Mar. 20, 2007, U.S. Appl. No. 11/361,325, 11
pages. cited by other .
U.S. Appl. No. 11/278,009, filed Mar. 30, 2006--189442/US/3. cited
by other .
Non-final Office Action dated Aug. 12, 2009, U.S. Appl. No.
11/278,009, 8 pages. cited by other .
Preliminary Amendment dated Dec. 11, 2007, U.S. Appl. No.
11/278,009, 6 pages. cited by other .
Preliminary Amendment dated Jun. 12, 2007, U.S. Appl. No.
11/278,009,9 pages. cited by other.
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Primary Examiner: Kreck; John
Attorney, Agent or Firm: Dorsey & Whitney LLP
Parent Case Text
PRIORITY CLAIM
This application is a Continuation-in-Part of U.S. patent
application Ser. No. 11/278,009 filed Mar. 30, 2006 and now
pending, which application claims priority to U.S. Provisional
Patent Application No. 60/732,511 filed Nov. 2, 2005. These
applications are incorporated herein by reference.
Claims
The invention claimed is:
1. A method for reducing chromium (VI) in an underground deposit of
chromium ore processing residue, comprising: injecting a ferrous
liquid into the underground deposit of chromium ore processing
residue, with the underground deposit comprising solid residue
particles, and with pore spaces between particles, with the ferrous
liquid reducing the pH of the pore water to less than about 7, and
with the volume of ferrous liquid injected sufficient to
substantially fill the pore spaces between the particles; waiting
until the pH of the injected ferrous liquid increases to a pH of
about 9; and then injecting a sulfide solution into the underground
deposit.
2. The method of claim 1 with the ferrous liquid comprising ferrous
chloride or ferrous sulfate, and with ferrous in the liquid
precipitating out as ferrous hydroxide with the increase in the pH,
and with the ferrous hydroxide then dissolving into the sulfide
solution and re-precipitating out as ferrous sulfide.
3. The method of claim 1 further comprising measuring the pH the
liquid in the deposit.
4. The method of claim 1 further comprising measuring pH of COPR
samples treated with the ferrous treatment chemicals and
determining time taken to achieve a desired pH.
5. The method of claim 1 further comprising waiting a predetermined
time interval.
6. The method of claim 1 wherein the ferrous liquid comprises
ferrous chloride or ferrous sulfate and the sulfide solution
comprises sodium sulfide.
7. The method of claim 1 with the ferrous liquid comprising ferrous
chloride or ferrous sulfate including up to %5 excess acid.
Description
BACKGROUND
For much of the twentieth century, chromite ore was processed at
various locations in the United States, to manufacture chromium and
related materials. Processing the chromite ore created large
amounts of chromite ore processing residue (COPR). Millions of tons
of COPR were then placed into the ground, often at or near the
processing locations. These sites, which are now contaminated with
COPR, are in or near densely populated urban and waterfront areas
in United States. There are similarly contaminated sites in Europe,
Japan, and other countries.
COPR is similar in texture to coarse gravel. It is formed as solid
nodules or pellets generally 1/4 to 1/2 inch in diameter, as a
waste product from ore processing. These pellets were often used
like gravel, as grading and fill material, and also in construction
of residential, commercial and industrial buildings. COPR was also
used in roadbeds and pipeline trenches. Consequently, some COPR
deposits may extend for thousands of feet under dense urban
development. In addition, in many of these locations, the COPR is
below the ground water table. In some locations, the COPR is mixed
with other materials, such as soil, sand, etc.
COPR is a strong alkaline or caustic material. It typically has a
pH of about 11-12. COPR typically also contains %1-%30 of
hexavalent chromium, having the chemical symbol Cr(VI). Cr(VI) is
toxic to humans. It can be absorbed into the body via the skin,
mouth or via inhalation. It is known to cause cancer and genetic
mutations. Consequently, COPR presents serious environmental and
public health hazards.
Cr(VI) is also present in other types of contaminated sites,
dissolved in ground water. The Cr(VI) may be the result of releases
from metals plating operations, from the application of Cr(IV)
corrosion inhibitors, and from landfills or other disposal
sites.
At COPR contaminated sites, the chromium is present in the solid
particles as well as in the ground water in the pores or spaces
between the COPR particles or pellets. Since Cr(VI) is soluble in
water, if the pore water is removed, the hexavalent chromium is
replaced by a slow diffusion or leaching of additional hexavalent
chromium from within the particles. As a result, pump and treat or
soil washing is ineffective or at least impractical for treatment
of COPR sites.
Cr(VI) in pore water can be converted to trivalent chromium, which
has the chemical symbol Cr(III), using remediating chemical
compounds. These compounds include soluble ferrous iron salts, such
as ferrous sulfate or ferrous chloride, or other similar
remediating compounds. Cr(III) is insoluble and relatively
non-toxic. Accordingly, if the Cr(VI) could be substantially
completely converted to Cr(III), the COPR at many sites could then
be safely left in the ground. However, with these chemical
remediation methods, the soluble remediating compounds tend to be
washed away by ground water movement relatively quickly.
Consequently, the conversion process expectedly does not last long
enough to clean up the site.
Other in-situ clean up processes use biological reduction of the
Cr(VI), with or without use of other remediating materials. In
biological clean up techniques, organic materials containing
bacteria and nutrients are mixed into the COPR contaminated soil.
However, in general, these types of biological reduction techniques
require a pH conducive for growth of bacteria, typically about 6.5
to 9.5. Consequently, biological techniques have required adding
large amounts of acid into the contaminated site, to lower the pH
to a level acceptable for growth of bacteria. The acid causes
destruction of the COPR particle structure. This can make future
handling of the COPR more difficult. The acid also generates large
volumes of carbon dioxide gas. In addition, placing large amounts
of acid into the ground can damage structures on or in the ground.
The disadvantages of the need for this use of acids has largely
prevented effective use of biological remediation techniques on
COPR.
In view of these problems, plans for permanent clean up of COPR
sites have largely contemplated excavation and removal of the COPR
material. This can require demolition, in-fill, and reconstruction
of buildings on the contaminated sites. Moreover, the excavated
material must still be remediated off site to convert the Cr(IV) to
Cr(III), before it can be placed in landfill or other final
disposal site. The costs, disruption, and delays associated with
excavation and removal of the contaminated material can of course
be enormous. Treating sites having dissolved Cr(IV) presents
similar problems. Accordingly, improved methods for cleaning up
COPR and dissolved Cr(IV) contaminated sites are needed.
SUMMARY OF THE INVENTION
In a first aspect, in a method for treatment of dissolved chromium
or COPR, ferrous sulfide is provided as a substantially insoluble
reducing compound material in the pores of the COPR or soil. The
ferrous sulfide accordingly substantially remains in place and is
not washed out by water movement or diffusion. Accordingly, the
ferrous sulfide is available when hexavalent chromium diffuses from
the COPR. Ferrous sulfide may advantageously initially be applied
as solutions of ferrous and sulfide salts, which can be injected
separately into the COPR deposit, and then combine to form a solid.
In liquid form, the reducing salts are easier to apply into the
ground. The distribution throughout the pores may also better in
comparison to applying a reducing compound in a solid form.
In one aspect, a ferrous salt solution is injected or pumped into
an underground COPR deposit, displacing the ground water from the
pores in the COPR deposit. The ferrous salt solution is acidic.
Consequently, the ferrous salt remains in solution. Over time,
e.g., 1-5 days, the acidic ferrous salt solution tends to be
neutralized by the alkaline COPR deposit. The pH of the solution
rises causing the ferrous salt to precipitate out within the pores
of the COPR deposit. A sulfide solution may then be separately
injected into the COPR deposit to form ferrous sulfide. The ferrous
sulfide remains substantially remains in the COPR deposit, reducing
Cr(IV) leaching out of the COPR particles to Cr(III). The level of
Cr(IV) is reduced to an environmentally acceptable low level.
Other objects, features and advantages will become apparent from
the following description. The invention resides as well in
sub-combinations of the steps and elements described. The steps and
elements essential to the invention are described in the claims,
other steps and elements being not necessarily essential.
DETAILED DESCRIPTION
In general, for treatment of COPR, the reducing compound should be
effective at reducing hexavalent chromium at a pH of about 8-13,
and typically about 10, 11, 12, or 13, so that the alkalinity of
the COPR does not need to be neutralized. This avoids the need to
add large amounts of acid to lower the pH. The reducing compound
advantageously generally does not excessively promote the formation
of minerals that can result in the swelling of the COPR. The
reducing compound is also preferably capable of remaining in the
pores for at least 6, 9 or 12 months, or longer, without loss of
effectiveness, even with movement of ground water. At some sites,
it may be necessary or advantages to have the reducing compound
remain in place for several years.
In one embodiment, a ferrous salt solution and a sulfide salt
solution (such as ferrous sulfate and sodium sulfide provided as
liquid precursors) are dispersed into the COPR contaminated zone.
The ferrous ions combine with the sulfide ions to form a colloidal
precipitate of ferrous sulfide. Since the ferrous sulfide particles
form in the injection system piping or in the soil, they are small
(colloidal) and hence easy to mix completely with COPR and
surrounding soil pores. The ferrous sulfide may be completely
solidified while still in the injection system, so that already
solid ferrous sulfide particles are placed in the ground. Ferrous
sulfide particles may alternatively be delivered in bulk to the
site, in solid or slurry form. Particles with a size of less than
about 5, 4, 3, 2 and more often 1 micron (mean diameter) are
generally more effectively injected in an aqueous liquid, in
comparison to larger size particles. The FeS particles are
consequently formed with an intended particle size of 1 micron or
less.
The ferrous sulfide reacts with hexavalent chromium in solution
converting the chromium to the trivalent form, which precipitates
as a hydroxide. The ferrous iron is oxidized and forms ferric
hydroxide precipitate. The sulfide is oxidized to elemental sulfur.
The low solubility of ferrous sulfide helps to prevent it from
being washed out of the system by groundwater movements. Ferrous
sulfate may be used with or instead of ferrous chloride.
The result of these reactions is the in situ lowering of the
hexavalent chromium in the water surrounding the COPR. Additional
hexavalent chromium will dissolve and diffuse from inside the COPR
particles to the particle surfaces, where it will react with the
solid ferrous sulfide particles. In addition, the ferrous sulfide
solids will partially dissolve releasing molecules of ferrous
sulfide which penetrate the COPR particles and react with dissolved
Cr(VI) in the COPR. Due to the low solubility of ferrous sulfide
only a small portion of the ferrous sulfide is dissolved as needed
for the Cr(VI) reaction. Hence the solid will remain for a long
time, unless needed for reduction of the Cr(VI). Sufficient ferrous
sulfide particles are provided to treat the hexavalent chromium
over a period of months or years to a desired remediation
standard.
The ferrous sulfide may be generated by the mixing of a ferrous
salt solution with a solution of sodium sulfide (above ground or in
injection system piping) by the following reaction:
FeCl.sub.2+Na.sub.2S---.fwdarw.FeS(s)+2Na.sup.++2Cl.sup.-.
The resulting precipitate of ferrous sulfide tends to form rapidly.
Consequently, the ferrous sulfide may solidify completely into
particles, before it is placed in the ground. The ferrous sulfide
generally will first form a neutral molecule of ferrous sulfide,
followed by growth to colloidal and larger particles of ferrous
sulfide. This makes it easier to inject and distribute throughout
the COPR if the ferrous sulfide is freshly precipitated when
compared to a solid that has to be reduced in size and injected as
a slurry. Additives such as surfactants, detergents, and phosphates
may be used. Precipitation slowing additives may also be used to
slow down formation of solid ferrous sulfide.
The FeS is advantageously formed as a solid either in the pores of
the COPR or in the pore space between individual COPR particles, or
in the equipment used to mix and inject the chemicals into the COPR
deposit. If formed on the outside of the pores, it is preferably
pushed uniformly throughout the pores of the COPR or the
subsurface. Excess ferrous sulfide is advantageously added to
account for oxidation by air, insufficient mixing, or other
losses.
Ferrous sulfide reacts with hexavalent chromium (represented as
chromate) by the following reaction:
CrO.sub.4.sup.-2+FeS+2H.sub.2O+2H.sup.+---.fwdarw.Fe(OH).sub.3+S+Cr(OH).s-
ub.3
Iron and chromium are converted to their trivalent form and
precipitate as hydroxides. Sulfide is oxidized to elemental sulfur
(not sulfate). This helps to avoid swelling, which appears to be
associated with mixing sulfate salts with COPR.
For stoichiometric reaction, for each gram of hexavalent chromium
(as Cr) need to add 1.08 grams of ferrous chloride (as Fe) plus 1.5
grams of sodium sulfide (as Na.sub.2S). Therefore add 3 times
stoichiometric of 3.24 g of ferrous chloride or ferrous sulfate (as
Fe) plus 4.5 g of Na.sub.2S for each gram of hexavalent chromium.
An FeS concentration greater than 3 times this stoichiometric dose
may be needed to provide good results. Commercial solutions of
ferrous sulfate and ferrous chloride may be used, as these contain
acid in addition to the salt. These materials are the byproduct of
acid pickling of steel. Accordingly, they are economically
available in large quantities. To minimize corrosion to chemical
delivery equipment, the excess acid may be neutralized with an
alkaline compound such as sodium hydroxide before injection.
Although the concentrations of the reducing compounds may of course
be varied for specific applications, the following guidelines may
be used.
Ferrous Chloride: 9 to 14% solution (as Fe) liquid technical
grade
Ferrous Sulfate: 5 to 7% solution (as Fe) liquid technical
grade
Sodium Sulfide: 10 to 30% solution (make from dry chemical)
The measurement of acceptable remediation of Cr(IV) may vary
depending on the characteristics, location, and regulation of each
specific contaminated site. A reduction of Cr(IV) to concentrations
of 240 to 20 mg/kg, or less, may be required, representing
reduction of 95% to 99.5% or more of the initial concentration of
Cr(VI) in the contaminated soil or COPR.
The ferrous sulfide may be injected or placed by pumping solutions
of the two chemicals separately with precipitation occurring in the
ground. When injected as a liquid, the reducing compound may be
placed into the ground with a hydro-punch or pipe, or with
injection wells, or using direct push methods. In a typical
application, a 1-4 inch diameter pipe is driven into position and
then the liquid is pumped in or injected. Injection times at each
punch or placement may vary, with 5-90 minutes being typical. The
pipe is then moved over to the next designated position. This
procedure can repeated, in a grid, spiral, or other pattern, until
the entire site has been injected. Slant injection may also be used
to place the liquid or slurry reducing compound under in or on
ground structures, or to reach positions not easily directly
accessible from vertically above. Hydraulic or pneumatic fracturing
methods may also be used, optionally in combined
fracturing/injection methods to deliver a slurry containing ferrous
sulfide particles to the in ground deposit. Fracturing has the
potential for improving delivery of the FeS into low permeability
formations. Permeability of fractured formations may be
dramatically increased, depending on the site conditions.
With injection methods for treatment of COPR, the FeS particles may
be formed by mixing of the FeCl.sub.2 and the Na.sub.2S solutions
into the injection equipment. Separate metering pumps may be used
for each component, with the solutions passing through an in line
mixer before injection. Since the reaction between the Fe.sup.2+
and the S.sup.2- is very rapid, small particles may be created.
Deflocculating and/or sequestering agents, such as polyphosphate,
non-ionic detergent, or silicone-based dispersing agents may be
added to help keep the FeS particles dispersed as they are
delivered into the underground matrix. Since the FeS is practically
insoluble in water, emulsified vegetable oil may be used as a
transport medium to disperse the FeS through the COPR. Caustics may
be added to neutralize the excess acids of the ferrous salt before
injection.
While it may not be necessary in most applications, the reducing
compound may also be placed in permanent, or semi-permanent wells
or well pipes. While most COPR deposits are below the water table,
the present methods may also be used in COPR deposits above the
water table. Similarly, these methods may be used to clean up
Cr(VI) contamination other than from COPR sites, above or below the
water table. In the case of COPR, since the reducing compound will
generally be mixed with a solution containing water before or as it
is placed into the COPR deposits, the pores between the pellets
will become filled with the ferrous sulfide containing liquid even
above the water table. Regardless of the type of contamination to
be treated, the ferrous salt, or the sulfide salt, or both, may
also be added to the soil as dry salts. Water in the ground
(natural groundwater or water pumped into the ground) may then mix
with the salt(s) in the ground. The salt(s) dissolve in the water,
mix together and chemically react to form solid ferrous
sulfide.
In augering applications, conventional or hollow stem augers may be
used. With augering, the reducing compound may be a solid, a liquid
or a slurry. Alternatively, components can be mixed in-line before
injection or mixed and injected using an auger soil mixer. Other
methods of mechanically mixing the soil with ferrous sulfide or
ferrous sulfide precursors may be used, including plowing,
rototilling, and soil excavation followed by above ground mixing
and then mixed soil replacement.
Testing was conducted on chromite ore processing residue (COPR).
Several columns were prepared to evaluate COPR chromium reduction
with various concentrations of sulfide along with either ferrous
chloride or ferrous sulfate. The columns were prepared in the
following manner:
1. Column material consists of 6-inch clear PVC pipe with white PVC
end caps.
2. The bottom end cap included a 1/2 inch plastic valve for
sampling the liquid phase of the column, and was sealed using PVC
glue.
3. The top end cap included two 1/4 inch barbed fittings for
filling and venting during set up and sampling, and was sealed with
an inert silicone based vacuum grease, allowing the top to be
removed for solids sampling.
4. Approximately 1-inch of geotextile material and approximately
4-inches of 0.2-mm quartz sand were added to the base of the column
to support the COPR material, and allow water to drain freely.
5. Deionized water was added to the columns to determine the pore
volume contained in the geotextile material and sand. This volume
was determined to be 900-ml. Two of these pore volumes will be
removed from the column before liquid samples are taken, which will
represent the liquid portion surrounding the COPR.
6. The COPR material was screened using a 0.5-inch sieve.
7. The stoichiometric amount of sodium sulfide was determined from
the Cr-VI concentration in the COPR. The sodium sulfide solid
material was weighed on an analytical balance and dissolved in
1-liter of deionized water.
8. The amount of iron product was determined based on the sulfide
and Cr-VI concentrations. Analytical grade ferrous chloride (powder
hydrated with deionized water) was used for column 1 (C1), and
technical grade ferrous chloride and ferrous sulfate liquid
material was used for the other columns.
9. The appropriate amount of screened COPR was placed in a 2-gallon
disposable plastic bucket and placed in a laboratory fume hood.
10. 1-liter of site groundwater was added to the COPR first, to
create a slurry.
11. 1/3 of the sulfide was added, mixed well, and then followed
with 1/3 of the ferrous iron and additional mixing. This process
was continued until all the treatment chemicals were added.
12. The COPR with treatment chemicals was then added to the test
columns.
13. The top end cap was sealed with vacuum grease and placed on the
column. Groundwater was added to fill the column and eliminate
headspace.
14. Table 1 summarizes the conditions used for each of the column
tests.
15. Sampling was started by allowing 1,800-ml to flow from the
column first. This represents two times the void volume contained
in the geotextile material and sand at the base of the column.
After this portion is removed, samples that represent the liquid
contained in the COPR material is collected for testing.
16. After the water samples are collected the top caps are removed
for solids sampling. A core device is used to collect a
top-to-bottom column of COPR material for testing.
17. After sampling the top cap was replaced, and the initial pore
water was returned to the column, along with additional groundwater
to eliminate headspace.
18. Analytical data for samples taken during the first 72 days
following chemical addition are presented in tables 2 and 3. Table
2 shows the pore water hexavalent chromium concentrations. Table 3
shows the hexavalent chromium in the solid COPR.
19. All doses of ferrous iron and sulfide reduced the pore water
concentration of hexavalent chromium in the pore water and in the
COPR solids within a 2 month period.
TABLE-US-00001 TABLE 1 Column Dose Calculations Dose for Each
Column Parameter Units C1 C2 C3 C4 C5 COPR amount Kg 5.0 5.0 4.0
4.0 4.0 COPR Cr-VI g/Kg 3.41 3.41 3.41 3.41 3.41 COPR Cr-VI g 17.05
17.05 13.64 13.64 13.64 COPR Cr-VI moles 0.33 0.33 0.26 0.26 0.26
Na.sub.2S * 9H.sub.2O (~100%) g 472 Sulfide, as S g 63 Sulfide, as
S moles 1.97 Na.sub.2S (60%) g 182 146 152 101 Sulfide, as S g 44.8
35.9 37.4 24.9 Sulfide, as S moles 1.4 1.12 1.16 0.77 FeCl.sub.2 *
4H.sub.2O (reagent) g 389 Fe.sup.2+ g 109 Fe.sup.2+ moles 1.96
Ferrous Chloride (Kemiron) 10.46% Fe.sup.2+ g 2,107 530 350
Fe.sup.2+ g 220 55 37 Fe.sup.2+ moles 3.95 0.99 0.66 Ferrous
Sulfate (Kemiron) 5.20% Fe.sup.2+ g 3,379 Fe.sup.2+ g 176 Fe.sup.2+
moles 3.15 Sulfide:Cr-VI ratio as S:Cr mole/mole 6.0 4.2 4.3 4.5
3.0 Iron:Cr-VI ratio as Fe.sup.2+:Cr mole/mole 6.0 12.0 12.0 3.8
2.5
TABLE-US-00002 TABLE 2 COPR FeS Column Test Results--Water Reaction
Time Cr-VI (ug/L) (days) C1 C2 C3 C4 C5 0 2,650 2,650 2,650 2,650
2,650 5 -- <1 <1 -- -- 14 <1 -- -- 8.44 9.02 42 -- -- --
-- -- 46 -- -- -- ND ND 68 -- ND ~7 -- -- 77 ~7 -- -- -- --
TABLE-US-00003 TABLE 3 COPR FeS Column Test Results--Solids
Reaction Time Cr-VI (mg/Kg) (days) C1 C2 C3 C4 C5 0 3,410 3,410
3,410 3,410 3,410 12 -- <0.010 <0.019 -- -- 14 -- -- -- 0.30
<0.11 21 0.13 -- -- -- -- 42 -- -- -- -- -- 46 -- -- -- 0.11
0.42 68 -- <0.053 <0.065 -- -- 77 0.42 -- -- -- --
While iron sulfide (FeS) can be formed using ferrous chloride
(FeCl.sub.2) and sodium sulfide (Na.sub.2S) solutions and
mechanically mixed with COPR deposits, this process requires direct
access to the ground surface and a significant disturbance of the
site. It therefore may be not well adapted for use on some
developed sites, unless the structures on the developed site are
first demolished and removed. Injection of the solutions avoids
these disadvantages of mechanical mixing. However, getting the
injected liquids to the desired location in the COPR deposit before
they precipitate as iron sulfide can be problematic, since the
resulting solid can plug up the pores in the solids preventing flow
and depleting the chemicals before they reach the COPR deposit.
COPR deposits have a high concentration of alkaline materials.
Water in the pores between COPR particles will typically have a pH
of about 11-12. Ordinarily, ferrous iron has low solubility in COPR
deposits, due to precipitation of ferrous hydroxide at high pH
conditions. Ferrous iron combines with hydroxide according to the
following reaction:
Fe.sup.+2+2OH.sup.-.fwdarw.Fe(OH).sub.2(solid)
At equilibrium, the following solubility equation applies:
[Fe.sup.+2].times.[OH.sup.-].sup.2=10.sup.-14.39
Where [Fe.sup.+2] denotes the molar concentration of ferrous
iron.
At pH 12 [OH.sup.-]=10.sup.-2 moles/L
[Fe.sup.+2]=10.sup.-14.39/(10.sup.-2).sup.2=10.sup.-10.31 1 mole/L
of [Fe.sup.+2]=55.85 g/l, Fe.sup.+2=0.0000027 mg/L at pH 12
At pH 7 [OH.sup.-]=10.sup.-7
[Fe.sup.+2]=(10.sup.-0.39)(55.85)=27.34 g/l or 3%
At pH 6 [Fe.sup.+2].sub.solubility=30%
Ferrous sulfate and ferrous chloride are produced by contacting
sulfuric or hydrochloric acid with ferrous iron deposits or steel.
The technical commercial chemical generated as a by product from
steel production has up to 5% excess acid (typically sulfuric acid
in ferrous sulfate and hydrochloric acid in ferrous chloride) mixed
with ferrous salt. It has now been discovered that when ferrous
chloride or ferrous sulfate (commercial or technical grade)
solutions are mixed with COPR in concentrations used for Cr (VI)
reduction, the pore water pH is depressed to about 6-7. It then
remains depressed typically for a few days. Then the pH rises back
to a pH of about 9-12.
In an alternative method for treating COPR deposits or formations,
ferrous chloride or ferrous sulfate solutions are injected into the
COPR deposits. The acid in the solution keeps the solution in the
pH range (e.g., 6-7) where ferrous iron is soluble. The ferrous
chloride or ferrous sulfate solution may be injected in a volume of
water sufficient to fill the pore volume in the target COPR
deposit, so that the ferrous will be distributed throughout the
deposit. After this injection of ferrous, the alkaline material in
the COPR neutralizes the acid in the ferrous solution. As the pH
rises, the ferrous in the ferrous solution precipitates' out as
solid ferrous hydroxide. This precipitation time interval typically
takes 1, 2, 3, 4, 5, or 6 days. Under certain conditions, it may
take even longer. The pH in the COPR deposit may be monitored via
sampling or underground probing to determine when the precipitation
time interval has elapsed. Alternately, samples of the COPR can be
mixed with ferrous solution and pH monitored to determine time for
the pH to reach a point where ferrous hydroxide is sufficiently
insoluble. Alternatively, the precipitation time interval can
simply be set at an interval sufficiently long (e.g., more than 7
days) to insure that the pH in the COPR deposit has returned to its
original range, and that the ferrous has precipitated out.
After the precipitation time interval has passed, a sodium sulfide
solution is injected into the COPR deposit. The sodium sulfide
solution is also provided in a volume sufficient to fill the pores
in the deposit (i.e., the open spaces between the COPR particles).
Since the ferrous hydroxide is at this time precipitated out as
solid, it is not significantly displaced by the sodium sulfide
solution. Rather, the ferrous hydroxide generally remains in place
within the deposit. Since ferrous sulfide is much less soluble than
is ferrous hydroxide, the ferrous iron tends to go into solution
and then re-precipitate out as ferrous sulfide. The ferrous sulfide
then reduces the Cr (VI) as described above.
Fe(OH).sub.2(solid)+2Na.sup.++S.sup.=.fwdarw.FeS.sub.(solid)+2Na.sub.4.su-
p.+2OH.sup.-
At pH 12, as shown before, Fe.sup.+2=0.0000027 mg/L
The solubility of FeS.sub.(solid) is governed by
Fe.sup.+2+S.sup.=.fwdarw.FeS.sub.(solid)
[Fe.sup.+2].times.[S.sup.=]=10.sup.-18.1
The resulting ferrous concentration at equilibrium with 1 mg/L of
S.sup.= At this concentration the molar concentration of sulfide
is: [S=]=10.sup.-4.5 [Fe.sup.=2]=10.sup.-13.6 Molar
Fe+2=0.0000000014 mg/L with 1 mg/L of S.sup.= present
Fe+2=0.0000027 mg/L in equilibrium with hydroxide.
Hence, even at pH 12, Fe(OH).sub.2 will dissolve and precipitate
FeS.
In addition, since the COPR inherently has a high pH, when treating
a mixture of COPR and other fill material (which typically has a
lower pH), the ferrous iron will tend to precipitate as
Fe(OH).sub.2 preferentially where the COPR is located, i.e., in the
high pH regions of the fill material. Consequently, the COPR is
essentially specifically and automatically targeted by the
resulting FeS precipitate.
Where the COPR deposit is below the water table, the spaces or
pores between the gravel-like COPR particles are filled with ground
water. In the pumping or injection process described above, the
ferrous liquid displaces the ground the water from the pores. The
sulfide solution then later displaces the ferrous liquid (after the
ferrous iron precipitates out due to the rising pH). The injection
process may be performed by installing temporary wells into the
COPR deposit, and pumping the chemical solutions (acidic ferrous
solution followed by sulfide solution) into the deposit via the
wells. The chemical solutions may also be pumped into one or more
wells, while ground water is pumped out of one or more other wells,
to aid in the displacement of the ground water and distribution of
the chemical solutions.
Depending on the size and characteristics of the COPR deposit to be
treated, each of the chemical solution may be pumped in for a time
interval ranging from minutes to days. The treatment zone within
the deposit will typically extend from about 5 or 10 feet, up to
about 100 feet, from the well hole. To achieve adequate
distribution of the chemical solutions throughout the deposit,
multiple well holes located in various patterns may be used. The
well holes may optionally be removed or closed after injection of
the chemical solutions. The chemical solutions may also optionally
be injected via direct push methods, i.e., by simply driving a pipe
into the COPR deposit, and pumping the chemical solutions through
the pipe and into the COPR deposit (while also optionally raising
the pipe through the deposit). Following injection of the chemical
solutions, reduction of Cr(IV) may go on for several months to
several years.
As used here, the singular includes the plural and vice versa,
unless specifically excluded by the context. The word "or" as used
here means either one, or any one, both, or all of the listed
items, and does not mean an alternative qualitatively different
element, or a non-equivalent element. The elements or steps
described relative to one embodiment apply as well to other
embodiments, except when otherwise specified.
Thus, novel methods and systems have been described. Various
changes and modifications may of course be made without departing
from the spirit and scope of the invention. The invention,
therefore, should not be limited, except to the following claims
and their equivalents.
* * * * *